In recent years, the development of silicon-based photomultipliers is being actively pursued. Along with that, radiation detecting devices, such as X-ray computer tomography (CT) scanners, that include a photomultiplier are also being developed. In an X-ray CT scanner, X-rays that have passed through a subject are detected, and sectional images (reconstructed images) of the subject are reconstructed in which CT numbers (CT values) corresponding to the attenuation fraction of the X-rays are treated as pixel values. Herein, CT numbers (CT values) differ according to the substance through which the X-rays pass. Hence, by generating a reconstructed image, it becomes possible to observe the internal structure of the subject. However, if the radiation dose of X-rays is small, then the reconstructed image includes an increased number of pixels causing noise because of having CT numbers (CT values) with errors. As a result, observation of the subject becomes a difficult task.
In order to solve such an issue, a technique is known in which, aside from a normal reconstructed image, a blurred reconstructed image is also generated in which noise does not occur easily. Then, the blurred reconstructed image is used in detecting the directions of edges of the substances; and smoothing in the detected directions of edges is performed with respect to the normal reconstructed image. With that, an image is obtained in which noise is reduced without causing defocussing of the edges. However, in such a smoothing operation, a simple mean filter or a weighted mean filter is used. Hence, although it is possible to reduce the spatial high-frequency components of the noise representing local irregularity, it is not possible to reduce the low-frequency components. For that reason, there remains unevenness in the image, and the accuracy of CT numbers (CT values) is not enhanced.